I. Field of the Invention
This invention relates generally to a dispenser for the controlled delivery of parenteral medication or interal solution into a patient and more particularly to a purely mechanical drug injection system for precisely controlling the flow rate of the medication from a syringe or vial into a patient.
II. Discussion of the Prior Art
Drug infusion has been around since the event of a long-term vascular access shunt which facilitates the continuous administration of intravenous antibiotics and nutritional solutions. To improve patient mobility and to increase the accuracy of the delivery rate, other means of drug infusion, such as electronic pumps and low-cost mechanical pumps have been devised. Electronic pumps, such as motorized syringe pumps, are accurate, but require a power source, such as batteries or a power line for its operation and they tend to be bulky and pole mounted. Comparable mechanical pumps are less accurate and tend to afford fewer features than are provided by electronic pumps. They do, however, offer an advantage of lower cost, greater portability and single-use disposability. Other known types of disposable infusion pumps, such as elastomeric pumps, have been developed for fixed flow rate applications.
Presently, with strong control by insurance companies on the treatment cost of diseases involving infusion, a low-cost reliable and disposable device, exhibiting a lower chance of infection, in comparison to other multiple use delivery devices are advantageous. Infusion therapies, such as pain management, chemotherapy and diabetes care, require higher infusion accuracy as well as stricter drug stability and biocompatibility requirements in comparison to antibiotic therapy.
The bio/drug compatibility problem becomes more of an issue for long-term, slow, infusion and applications requiring low volume of drug, such as insulin, where the ratio of drug contacting surface to drug volume is high. Similarly, therapies requiring drugs at low concentration, drugs with very short half-life, drugs soluble in lipid or high polar solutions provide insurmountable challenges for selection of material that contacts the drug. Similarly, applications such as inter-muscular drug infusion applications, require the highest level of bio-compatibility and the lowest level of leachability for drug contact material. Bio-compatibility tests are expensive and tend to be quite time consuming and must be performed on all material contacting the drugs in the delivery device. Syringe pumps, using approved syringes and administration sets require minimal bio-compatibility and drug stability and leachability testing. Syringe pumps which interfere with drug flow for the purpose of controlling the flow, such as spring-powered syringe pumps with capillary rate control, require material bio-compatibility and drug stability and solution leachability testing for materials used in the rate control element, connectors and non-standard administration line.
Elastomeric pumps with high elastomeric surface contact, as well as numerous parts that contact the drug must undergo additional material tests. The material used in drug contacting flow control elements must also pass compatibility testing. The delivery platforms where the drug is metered by a flow control element, such as spring-loaded syringe pumps, and almost all elastomeric-powered pumps, further suffer from dependence of delivery accuracy on physical properties of the drug. The drug solution's physical properties, such as viscosity or density, can cause delivery rate changes due to changes in drug concentration or changes in the use environment, such as temperature changes. Prior art mechanical disposable syringe pumps provide a constant delivery rate by either providing a constant velocity motion to the plunger of a syringe by some clock-type mechanism, such as is described in U.S. Pat. No. 4,602,700 or by pressurizing the drug in the syringe and placing a flow resistance element in the delivery flow circuit. Here, reference is made to U.S. Pat. Nos. 4,289,006, 4,381,006 and 4,755,172. Such clock-type mechanisms tend to be complex, typically have many moving parts and are not capable of providing a driving force to overcome frictional resistance encountered in larger syringes. A disposable syringe driver that controls the flow rate of pressurized drugs requires a flow restriction element in the drug delivery circuit. Such an arrangement exhibits the further disadvantages of mandating a non-standard administration line in order to adapt to the configuration of the flow resistance element. Moreover, dependence of the delivery rate on the type of drug and its concentration. Further, such prior art devices exhibit a variation in drug delivery rate with environmental changes, such as temperature, which may impact the viscosity of the drug and, therefore, its flow rate. For these reasons, a need exists for an improved drug delivery apparatus that obviates most, if not all, of the foregoing problems is needed. It is the primary object of the present invention to provide just such a device.